Obstructive Sleep Apnea is now recognized as one of the most common disorders in the U.S. The lower oxygen levels associated with Obstructive Sleep Apnea is now known to be a major cause of cardiovascular morbidity including heart attack and stroke. A crisis exists in the U.S. in that traditional expensive polysomnography cannot be used to identify these patients on a sufficient scale. The situation is analogous to having a disease as common and subtle as insulin dependent diabetes without an inexpensive and widely implementable and simple mechanism to diagnose the disorder (such as exists for diabetes). Millions of patients remain undiagnosed. The development of a diagnostic system which can allow simplified diagnosis of obstructive sleep apnea by the primary care physician is a national healthcare priority of substantial scale. The prevention of hundreds of thousands of annual excess deaths, stroke and heart attacks associated with obstructive sleep apnea through simplified recognition of this disorder is the most important purpose of the present invention. These excess deaths are occurring annually in a great part due to the lack of availability of this technology resulting in a vast pool of undiagnosed cases of Sleep Apnea. Despite the fact that obstructive sleep apnea is easily treated, both the patient and the family are often completely unaware of the presence of this dangerous disease, thinking the patient just a "heavy snorer".
Obstructive sleep apnea often develops insidiously as a patient enters middle age and begins to snore. The major cause is an increase in fat deposition (often age related) in the neck which results in narrowing of the airway. (In fact the probability that a 40 year old has sleep apnea is directly related to his or her neck circumference). When the muscle tone of the upper airway diminishes during sleep and negative pressure associated with inspiration through this somewhat narrow airway results in collapse of the upper airway in a manner analogous to the collapse of a cellophane straw. This results in airway obstruction and, effectively chokes off all air movement The choking patient (still asleep) begins to struggle and inhales more forcibly, thereby, further lowering upper airway pressure and causing further collapse of the upper airway. During this time, substantially no air movement into the chest occurs and the patient experiences a progressively fall in oxygen (similar to the fall occurring early in drowning). The fall in oxygen produce central nervous system stimulation contributing to hypertension and potential heart and blood vessel injury and finally results in arousal. Upon arousal, increase in airway muscle tone opens the airway and the patient rapidly inhales and ventilates quickly to correct the low oxygen levels. Generally, the arousal is brief and the patient is not aware of the arousal (or of the choking since this occurs during sleep). Once oxygen levels have been restored, the patient begins again to sleep more deeply, upper airway tone again diminishes, the upper airway collapses and the cycle is repeated stressing the heart with low oxygen in a repetitive fashion. Often this repeating cycle over many years eventually results in damage to the heart muscle and/or the coronary arteries. As the patient ages, the consequences of undiagnosed obstructive sleep apnea is often either a progressive decline in heart muscle function (and eventual heart failure) or heart infarction.
The duration and severity of each apnea is quite variable from patient to patient and with the same patient throughout the night. Indeed, the disease process represents a spectrum of severity from mild snoring, which is associated with incomplete and inconsequential airway obstruction, to severe apneas which can result in fatal hypoxemia.
This disease commonly results in excessive daytime sleepiness and can disrupt cognitive function during the day due to fragmentation of sleep during the night associated with recurrent arousals of which the patient is not aware.
Although this disease commonly affects obese patients, it may occur in patients with any body habitus. Because this disease is so common and because it presents with the subtle and common symptoms of excessive daytime sleepiness, morning headache, and decreasing ability to concentrate during the day, it is critical that an inexpensive technique for accurately diagnosing and treating this disease be developed. Traditionally, this disease has been diagnosed utilizing a complex and expensive multi-channel polysomnogram. This is generally performed in a sleep lab and involves the continuous and simultaneous measurement and recording of an encephalogram, electromyogram, extraoculogram, chest wall plethysmogram, electrocardiogram, measurements of nasal and oral air flow, and pulse oximetry. These, and often other, channels are measured simultaneously throughout the night and these complex recordings are then analyzed to determine the presence or absence of sleep apnea.
The problem with this traditional approach is that such complex sleep testing costs between one-thousand to thirty five hundred dollars. Since sleep apnea is so common, the cost of diagnosing obstructive sleep apnea in every patient having this disease in the United States would exceed Ten Billion Dollars. It is critical that a new, inexpensive technique of accurately diagnosing sleep apnea be developed.
Nocturnal oximetry alone has been used as a screening tool to screen patients with symptoms suggestive of sleep apnea to identify whether or not oxygen desaturations of hemoglobin occur. Microprocessors have been used to summarize nocturnal oximetry recordings and to calculate the percentage of time spent below certain values of oxygen saturation However, oxygen desaturation of hemoglobin can be caused by artifact, hypoventilation, ventilation perfusion mismatching. For these reasons, such desaturations identified on nocturnal oximetry are not specific for sleep apnea and the diagnosis of sleep apnea has generally required expensive formal polysomnography.
The present invention comprises a system and technique for deriving and utilizing the analysis of graphical pulse oximetry-derived waveforms as a function of time to accurately diagnosis sleep apnea with adequate specificity to, in many cases, eliminate the need for expensive formal polysomnography.
It is the purpose of this invention to provide an inexpensive system for the collection and analysis of pulse oximetry values as a function of time during sleep to provide a diagnosis of sleep apnea with a high degree of specificity.
This invention provides a reliable and specific means for the diagnosis of obstructive sleep apnea which can be performed in the patient's home without attendance of technical personnel. It is further the purpose of this invention to provide an inexpensive and accurate means to both screen for and specifically diagnose obstructive sleep apnea by a single overnight recording in the patient's home without the need for multiple connections to different parts of the patient's body. It is further the purpose of this invention to define a technique for diagnosing obstructive sleep apnea utilizing the calculation of the ascending and descending slope ratio of phasic oxygen desaturations measured during sleep.
Specifically, the present invention defines a device for diagnosing sleep apnea, that has the following components. First, a means must determine an oxygen saturation of a patient's blood. This saturation value is coupled to a means for identifying a desaturation event based on the saturation value. The desaturation event is one in which said oxygen saturation falls below a baseline level by a predetermined amount and for a predetermined time. The slope of the event is calculated by means for calculating a slope of said desaturation event representing a rate of change per unit time of fall of oxygen saturation. This slope is used by a means for comparing said calculated slope with a value of slope which is determined in advance to be indicative of sleep apnea, and determination of diagnosis of sleep apnea is made based on said comparing.
The comparing can be done by:
1) comparing with an absolute number which is likely to indicate a sleep apnea, or PA1 2). comparing with other slopes taken at different times.
The identifying means can also identify a resaturation, immediately following said desaturation and coupled with said desaturation, in which the oxygen saturation rises, and wherein the determination can also be based on a slope of said resaturation.
Many other ways of calculating the slope are also disclosed herein.
These and other aspects of the invention will now be described in detail with reference to the accompanying drawings, wherein: